Material processing is used in almost every industry to produce materials of varying properties for products of varying application. In some areas, methods of material processing are still developing. This includes the area of shape memory materials.
Shape memory materials are materials that can be trained to hold and return to a particular shape when at a higher temperature and be malleable at a lower temperature. Even if bent into a different shape when at the lower temperature, the material returns to the trained shape when the temperature is raised. The temperature at which the material reverts back to the trained high temperature configuration is typically referred to as the transformation temperature. The shape memory effect that occurs in these materials is related to a reversible solid state phase transition in which the material transforms between an austenitic state and a martensitic state with a decrease in temperature. In the martensitic state, the shape memory material becomes more easily deformed and is typically able to accommodate significant plastic deformation at an almost constant stress level. When the shape memory material is in the martensitic state, it can be heated and the application of heat results in the metal returning to the austenitic state. The transformation may occur at a particular temperature or over a range of temperature. Shape memory materials have become quite well known and are used in many applications such as medical (e.g. stents), industrial, automotive, aerospace and various others.
Shape memory materials can be generally divided into shape memory metals/alloys (SMAs) and shape memory polymers (SMPs). Many alloys may be manipulated into a shape memory material, including some magnetic materials and alloys. Three main types of SMAs include:
1) Nickel-titanium (NiTi)
2) Copper-Zinc-Aluminum-Nickel
3) Copper-Aluminum-Nickel
Other SMAs include, but are not limited to, the following:
1) Ag—Cd 44/49 at. % Cd
2) Au—Cd 46.5/50 at. % Cd
3) Cu—Al—Ni 14/14.5 wt. % Al and 3/4.5 wt. % Ni
4) Cu—Sn approx. 15 at. % Sn
5) Cu—Zn 38.5/41.5 wt. % Zn
6) Cu—Zn—X (X═Si, Al, Sn)
7) Fe—Pt approx. 25 at. % Pt
8) Mn—Cu 5/35 at. % Cu
9) Fe—Mn—Si
10) Pt alloys
11) Co—Ni—Al
12) Co—Ni—Ga
13) Ni—Fe—Ga
14) Ti—Pd in various concentrations
15) Ni—Ti (˜55% Ni)
(at. %=atomic percent)
Examples of SMPs include, but are not limited to, the following:                1) Polyurethane-based shape-memory polymers with ionic or mesogenic components        2) Polyethylene-terephthalate-Polyethyleneoxide (PET-PEO) block copolymer crosslinked using Maleic Anhydride        
One of the most common shape memory materials is nitinol (sometimes referred to as NiTi), an alloy of nickel and titanium. This application focuses on SMAs and nitinol in particular, however, similar principles can apply to other SMAs, SMPs or shape memory materials, as will be understood by one skilled in the art.
SMAs are typically monolithic materials that are capable of a single transformation temperature. The physical properties of SMA's, including elasticity and stiffness, are affected by a variety of factors including the chemical composition of the SMA and the particular treatment to which the SMA is subjected. In particular, for a nitinol SMA having slightly varying near-equiatomic base metal compositions, the ratio of NI to TI can significantly affect the transformation temperature.
The excellent pseudoelasticity, shape memory and biocompatibility of nitinol have made it a leading candidate for various applications, including aerospace, micro-electronics and medical devices. Its pseudoelastic properties enable nitinol to experience up to 18% strain and subsequently fully recover upon release. The shape memory effect results from nitinol's ability to transform from a rigid high temperature austenite phase to a malleable low temperature martensite phase during cooling. Once a high temperature shape is trained into a nitinol workpiece in the austenite phase, it can then be cooled to its martensite phase and be elastically deformed; however upon heating, the material will transform back into the austenite phase and return to its original shape. Primary factors affecting the transformation temperature include 1) alloying elements (i.e. the Ni to Ti ratio), 2) thermo-mechanical processing and 3) precipitates embedded in the metal matrix.
While the properties of nitinol with one transformation temperature are quite well known, more recently, efforts have been made to produce monolithic nitinol that has more than one transformation temperature in order to broaden the range of applications for SMAs and to make them more useful in existing applications.
The applicants are aware of two material forming techniques under development that are intended to be used to form monolithic shape memory alloys from base elements to provide an SMA having multiple transformation temperatures.
1) Tape Casting utilizes varying compositions of elemental powders and sinters them to form a monolithic material. Sintered near equi-atomic nickel and titanium powders have recently exhibited shape memory effects. Furthermore, attempts to vary local compositions on a monolithic sheet have been demonstrated. However the inherent nature of titanium to oxidize makes it extremely difficult to control the actual composition and the process can form a brittle structure. In addition, the porous material formed during sintering generally results in poor mechanical properties.
2) Laser engineering net shaping (LENS) is a commercially available rapid prototyping process, which uses elemental powders to create a layer by layer structure. By varying process parameters, it may be possible to modify transformation temperatures during processing. However, complexities associated with processing can make it difficult to accurately tailor transformation temperatures. In addition, the final product typically has a coarse surface finish and can require considerable post-processing.
Based on the foregoing, there is a need for improved methods and systems for processing or treating materials and, in particular, shape memory materials in order to provide a material with multiple transformation temperatures and attempt to overcome at least some of the concerns described above.